1. Field of the Invention
The present invention relates to a circuit device manufacturing method, and particularly to a circuit device manufacturing method in which an entire circuit board of a relatively large size, inclusive of its bottom surface, is sealed by resin. More particularly, the present invention relates to a resin sheet and a circuit device manufacturing method using the resin sheet. Still more particularly, the present invention relates to a circuit device in which a principal surface of a semiconductor element is coated with a thin coating of sealing resin, and a method of manufacturing the same.
2. Description of the Related Art
Methods for sealing a circuit board having a hybrid integrated circuit formed of a transistor or a chip element, embedded therein on a top surface of the hybrid integrated circuit, include a sealing method using a casing member, and a resin sealing method using resin.
With the use of the casing member, the casing member in the form of lid having a hollow portion is fitted to the circuit board, whereby the hybrid integrated circuit formed on the top surface of the circuit board is housed in the hollow portion of the casing member.
With the adoption of the resin sealing, injection molding using a molding die is employed to apply a coating to the hybrid integrated circuit formed on the top surface of the circuit board. Description will be given with reference to FIG. 19A with regard to a configuration of a resin-sealed hybrid integrated circuit device 400. In the hybrid integrated circuit device 400, first, the top surface of a circuit board 401 made of metal such as aluminum is wholly coated with an insulating layer 402. Then, a circuit element is connected to a conductive pattern 403 formed on the top surface of the insulating layer 402 thereby to form a predetermined hybrid integrated circuit. A semiconductor element 405A and a chip element 405B connected together by a fine metal wire 407 are depicted as the elements arranged on the top surface of the circuit board 401. At each end of the circuit board 401, a lead 404 is fixed to the conductive pattern 403 in the form of pad.
A sealing resin 406 is a thermoplastic resin, and coats the top, side and bottom surfaces of the circuit board 401. Here, a reduction in the thickness of the sealing resin 406 coating the bottom surface of the circuit board 401 is effective for good outward radiation of heat via the circuit board 401, the heat produced by the circuit element formed on the top surface of the circuit board 401. However, if the thickness of the sealing resin 406 coating the bottom surface of the circuit board 401 is set thin, for example on the order of 0.5 mm, there arises the problem that the bottom surface of the circuit board 401, in part, is not coated with the sealing resin 406. The reason is that in a process for injection-molding the sealing resin 406 by use of a molding die, a gap between the bottom surface of the circuit board 401 and the bottom surface of an inner wall of the molding die is so narrow that the sealing resin does not spread well throughout the gap.
A method for avoiding this problem will be described with reference to FIG. 19B. Here, the injection molding takes place with the circuit board 401 supported from underneath by a supporting member 410. Specifically, the supporting member 410 is made of a thermoplastic resin, and its inside surface is of such a size as to fit to the bottom surface and part of the side surfaces of the circuit board 401. Also, an outside surface of the supporting member 410 is of such a size as to come in contact with the bottom and side surfaces of the inner wall of a molding die 412. Thus, when the circuit board 401 supported by the supporting member 410 is placed in a cavity 414 of the molding die 412, the supporting member 410 is situated in the gap between the bottom surface of the circuit board 401 and the bottom surface of the inner wall of the molding die 412. Under this condition, the thermoplastic resin is injected into the cavity 414 thereby to provide the resin sealing for the circuit board 401. According to this method, the supporting member 410 is situated in the gap between the bottom surface of the circuit board 401 and the bottom surface of the inner wall of the molding die 412, and thus, there is no need for injection of a thermosetting resin in liquid form into the gap, which in turn prevents the occurrence of a void resulting from the bottom surface of the circuit board 401 being in a partially uncoated state. Also, a potting resin 420 is formed on the top surface of the circuit board 401 so as to coat the circuit element, for the purpose of protecting the fine metal wire or the like against injection pressure at the time of the resin sealing.
A resin sealing method for a semiconductor device of a lead frame type will be described with reference to FIGS. 20A and 20B. FIG. 20A is a cross-sectional view showing a resin sealing process, and FIG. 20B is a cross-sectional view showing a configuration of a circuit device 500 as manufactured.
Referring to FIG. 20A, an island 502 having a semiconductor element 504 fixed on a top surface thereof is placed within a cavity 514 formed by bringing an upper die 524 and a lower die 526 into contact with each other. Also, a pod 520 that communicates with the cavity 514 via a runner 518 is formed in the lower die 526, and a tablet 528 is placed in the pod 520. The tablet 528 is formed by pressure-molding a granular thermosetting resin, filler and the like, and has a cylindrical shape.
The above-mentioned molding die is heated to or above a temperature at which the tablet 528 placed in the pod 520 melts, and thus, the tablet 528 placed in the pod 520 is gradually melted into a sealing resin in liquid form. Then, the sealing resin in the liquid form pressed by a plunger 522 is fed to the cavity 514 via the runner 518 and a gate 516, and the semiconductor element 504 and the island 502 are sealed by the sealing resin. Also, with injection of the sealing resin, air in the cavity 514 is released to the outside through an air vent 556.
FIG. 20B shows the circuit device 500 resin-sealed by the above-mentioned process. A sealing resin 508 is used to resin-seal the island 502, the semiconductor element 504, a fine metal wire 506 and a lead 510. Also, the back surface of the island 502 is wholly coated with the sealing resin 508 in order to ensure pressure resistance and moisture resistance.
Nowadays, as electronic equipment such as a mobile telephone becomes smaller in size and becomes more multifunctional, a circuit device for use in such equipment is likewise desired to become still smaller in size.
A packaging structure for a highly integrated circuit element has shifted from conventional quad flat package (QFP) to chip size package (CSP) or wafer level package (WLP). With the CSP, the package is formed by dicing a wafer having external connection electrodes formed on one principal surface. Thus, the CSP is fixable, in size equivalent to the circuit element, to a packaging board, which in turn enables miniaturization of the packaging board on which the CSP is mounted. Hence, the adoption of the CSP also enables miniaturization of a whole set such as the mobile telephone.
A circuit device 600 of a conventional CSP type will be described with reference to FIG. 21. The circuit device 600 includes a circuit element 601 made of a semiconductor substrate having a diffusion region formed on a top surface, a protective film 602 coating the top surface of the circuit element 601, and an external electrode 607. An electrode 604 is a metal film formed in an opening formed by partially removing the protective film 602, and is connected to the diffusion region formed on the circuit element 601. The external electrode 607 is a bump electrode made of gold or a solder ball made of solder, and is fixed on the top surface of the electrode 604. A PSG (phospho-silicate-glass) film, a Si3N4 (silicon nitride) film, or the like is used as the protective film.
Referring to FIGS. 19A and 19B, methods for forming a sealing resin using a molding die include transfer molding besides the above-mentioned injection molding, and the transfer molding uses a thermosetting resin such as an epoxy resin to coat the top, side and bottom surfaces of the circuit board 401. In recent years, the transfer molding has come into more frequent use than the injection molding. The reason is that the transfer molding enables resin sealing at a lower temperature with lower pressure, and hence causes less adverse influence on a circuit element such as a semiconductor element, than the injection molding.
The transfer molding also uses the molding die 412 as shown in FIG. 19B for resin sealing. Therefore, a reduction in the thickness of the sealing resin 406 coating the bottom surface of the circuit board 401 may lead to the problem that the sealing resin is not filled into the gap between the bottom surface of the inner wall of the molding die 412 and the bottom surface of the circuit board 401. In addition, the injection molding uses a thermoplastic resin such as polyphenylene sulfide (PPS) for resin sealing of the circuit board 401. Therefore, it is difficult to apply the technology related to the injection molding to the transfer molding without any modification because of great variation in property between the thermoplastic resin used in the injection molding and the thermosetting resin used in the transfer molding.
Referring to FIGS. 20A and 20B, further, the above-mentioned sealing method poses the problem that the bottom surface of the island 502 is not coated. Specifically, a reduction in the thickness of the sealing resin 508 coating the bottom surface of the island 502 is desirable for good outward radiation of heat produced by the semiconductor element 504 via the island 502 and the sealing resin 508. A reduction in the thickness of the sealing resin 508 coating the bottom surface of the island 502 to about 0.5 mm or less, for example, achieves an improvement in heat radiation characteristics of the whole device. To this end, however, a gap between the bottom surface of the island 502 and an inner wall of the lower die 526 needs to be set narrow in the resin sealing process as shown in FIG. 20A, and such a narrow gap may not allow the sealing resin to be fully filled into the gap. If there is an unfilled region that is not filled with the sealing resin, the region forms a void, thus causing the occurrence of a defective condition.
In this regard, an increase in pressure applied to the sealing resin, at the time of injection of the sealing resin into the cavity 514, may possibly permit the filling of the sealing resin into the narrow gap under the island 502. However, the increase in the injection pressure exerted on the sealing resin can possibly cause the breaking of the fine metal wire 506 having a fine diameter on the order of a few tens of micrometers.
Referring to FIG. 21, further, the circuit device 600 of the above-mentioned configuration poses the problem that the circuit element 601 made of a semiconductor material such as silicon may have a crack in a process for transporting the circuit device 600 or in a process for mounting the circuit device 600 because the bottom surface of the circuit element 601 is exposed to the outside.
The problem of the occurrence of a crack in the circuit element 601 as mentioned above is relieved by coating the bottom surface of the circuit element 601 with a protective film such as resin. For this purpose, a protective film may be formed on the bottom surface of the circuit element 601 by using a coating method such as potting. In this case, however, the formation of a thin and even coating as this protective film is difficult. The formation of a thick coating of the protective film leads to the problem of increasing the thickness of the circuit device 600 as a whole, or the formation of an uneven coating of the protective film leads to the problem of impairing evenness of the circuit device 600.
An object of the present invention is to provide a circuit device manufacturing method which includes: integrally sealing a circuit board with a sealing resin made of a thermosetting resin; and coating the bottom surface of the circuit board with a thin coating of the sealing resin. Another object of the present invention is to provide a resin sheet and a circuit device manufacturing method using the resin sheet, the resin sheet allowing prevention of occurrence of a void at the time of resin sealing. Still another object of the present invention is to provide a circuit device manufacturing method capable of coating a principal surface of a circuit element with an extremely thin and even coating of resin.